QTR-Quadtone RIP

Hi Roy,

Thank you so much for responding, and confirming that:

1. The linearization targets should be printed "raw" without
   any color management.

2. The linearization results (after inserting the linearize
   line to the source text file of the curves, remaking and
   installing the curves, and restart Photoshop) shoule be
   also measured "raw" without color management, for the
   objective of Lab linearity.

Since my densitometer is borrowed, I have been concentrating
in understanding the linearization process.  As you pointed
out, what really matters is how the real prints look, or match
the display on screen on different types of paper, the area
I have not explored yet.  It is then more in the area of
artistic preference, or personal taste, wheather to use the
QTR Matte/Photo Paper curves for printing, which is no more
in the domain of analytical discussion.  Some of my friends
have told me they have been happy printing with QTR curves
without the profile conversion, but some may perfer to do so,
I guess.

The only question remaining right now to me is that, with the
only tool I have to exam the ICC profiles, the OS X ColorSync,
your QTR Gray curves simply contain a gray transfer curve that
do not look linear.  They are probably not pure explential,
but I measured the interceptions and found it is close to
a Gamma 2.5 curve from calculations.  I put the curve side
by side to the Gamma 2.2 and 1.8 curves trying to understand
the differences, and certainly they look much closer to the
2.2 curve, with the exception of the small interception at
near the origin in the Matte Paper profile.  Then I measured
the difference of the QTR curve relative to Gamma 2.2, and
predicted it would crush in some extremely low shadows to
pure black (from the interception), but expand the other
shadow values, and compress the mid-highlight a little bit so
the prints would be a bit brighter in midtones.  Next, I
watched the spiked histogram line shifts with a gray step
wedge image before and after converting (from the untagged
file under my working space that is Gamma 2.2, to the QTR
Matte Paper profile), that confirmed these predictions.

If a profile is identical to Gray Gamma 2.2, for instance,
the conversion from the Gray Gamma 2.2 to it does nothing.
If your profile is a "linear" curve, a conversion to it from
the Gamma 2.2 work space changes the numbers a lot before they
are fed to the QTR engine.  Since converting to the QTR
profile in your specified QTR printing workflow only changes
the print's looking little bit, as you described, can we say
that the profiles are closer to the Gamma 2.2 ones than being
"linear", even they are linear to the Lab values?

Many thanks again,
Leping

--- In QuadtoneRIP@yahoogroups.com, Roy Harrington <roy@...> wrote:
>
> Hi Leping,
> 
> You've got a lot of inter-related issues going on and I think
> they tend to confuse the situation.
> 
> In QTR there are two similar but different features.   First,
> there's the Linearization in the QTR profiles and second,
> there's the ICC profiling with either generic QTR Gray
> Matte/Photo Paper profiles or custom Create-ICC profiles.
 
> QTR profiles don't use a gamma function which is a pure
> exponential.  QTR linearization uses the Lab L* function.
> The idea being to make a straight-line in L measurements.
> The line always goes from dMin to dMax and the goal is a
> simple straight-line.  xGamma functions are a different shape.

> This is specifically for printing targets that will be used for
> linearization or ICC profiles.  You need to get "raw" density
> measurements so that a curve can be fitted to correct/linearize
> the output.

> It sounds like what you are doing is measuring a stepwedge for
> Lab linearity.  Since Lab linearity is what the basic QTR profile
> goal is then naturally you'll get the best linearity with just the
> basic QTR profile and No Color Management conversions.
> 
> Color Management and ICC profiles however have a different
> purpose and goal.  It's much harder to measure the precise result
> because the goal is matching of our perception not a specific
> linearity.
> 
> When you print on matte paper there's a mapping of dMin's and
> dMax's from the file to the paper.  In general you lose more at
> the shadow/dMax end.  So with straight QTR profiles this will
> usually mean a lighter print because the straight-line pushes
> the whole range lighter.  The idea of Color Management is to
> do a mapping that better (but not perfectly) matches our
> perception.  This sacrifices some of the shadow separation
> in favor of midrange density.
> 
> In addition to this issue you also have to consider the source
> embedded profile (or working space for untagged files).  Say
> you have a stepwedge and look at K=95, it's really the
> corresponding L value that determines how dark it is.  So for
> Gamma 2.2 there is very little separation between K=100 and
> K=95.  Look at the eye-dropper in Photoshop, the difference
> is only L=0 to L=1 -- much less than the separation between
> any other steps.
> 
> The bottom line with all color management issues is how good
> the print and the screen match with our perception not
> measurements with a densitometer.
> 
> Roy